1. Field of the Invention
The present invention relates to a single crystal pulling apparatus using Czochralski method and more particularly to a single crystal pulling apparatus having a screen for preventing a contamination of a crystal with impurities and enabling a single crystal rod to be pulled at high speed without blocking monocrystallization processes.
2. Description of the Related Art
Unexamined Japanese patent application publication No. 62-138386 discloses a single crystal pulling apparatus with an improved pulling speed in which a radiation screen is provided near the surface of a melt, the screen being of multi-layered insulating structure and having the shape of a substantially inverted cone, the surface of the screen being made of a sintered ceramic material or ceramic coated. The screen serves to block heat radiated from the melt to prevent a single crystal under pulling from being heated, thereby to increase the pulling speed while preventing contamination of the single crystal.
In addition, unexamined Japanese patent application publication No. 63-2566593 discloses a single crystal pulling apparatus comprises a screen with a section in the shape of an inverted trapezoid, which surrounds a single crystal pulling zone. The apparatus is provide with a cooling device to improve the operation of the screen.
These conventional apparatuses involve the following problems:
1) To increase pulling speed of a grown crystal, it is important to quickly dissipate, from the grown crystal, heat conducted from the melt, latent solidification heat generated in crystallization process, and heat radiated from heat sources such as the melt and a heater. In particular, when a temperature gradient at and around a solid-liquid interface is large, the pulling speed can be increased even if the cooling speed (i.e., the temperature of the grown crystal) is slightly low. PA1 2) When the radiation screen whose inner surface is made of a material having a high thermal emissivity and a high thermal conductivity such as carbon or silicon carbide (SiC) is used with a cooling device, the inner surface of the radiation screen absorbs heat radiated from the grown crystal so as to heat the radiation screen to a high temperature. As a result, the grown crystal receives heat radiated from both the screen and the grown crystal itself reflected by the screen and holds the heat therein. Thus, it is difficult to achieve a further increase in the pulling speed. PA1 3) When the radiation screen whose inner surface is made of a material having a high reflectivity is used with a cooling device, the screen reflects heat radiated from the grown crystal and the reflected heat is held in the grown crystal, which suppresses a further increase in the pulling speed. PA1 4) Since the radiation screen has a sectional shape of the inverted trapezoid or inverted cone and is placed remote from the surface of the melt and the wall of a crucible both radiating a large amount of heat and the radiated heat is insulated near the grown crystal, it cannot achieve a sufficiently cooling to the grown crystal. Thus, a further increase in the pulling speed is suppressed. PA1 1) The radiated heat reflection surface provided in a lower portion of the cooled cylinder is made of a material having a metal groove or layer. If a heavy metal is used for the material, the heavy metal is likely to contaminate a single crystal such as silicon single crystal during pulling of the single crystal. Thus, electrical properties of a device having the single crystal is deteriorated.
However, the screen in the shape of, for example, an inverted cone does not have sufficiently small solid angle for a grown crystal, especially at the solid-liquid interface. Therefore, the screen cannot dissipate all of heat radiated from the solid-liquid interface and from its vicinity. Thus, this conventional apparatus is difficult to achieve a further pulling speed increase.
Unexamined Japanese patent application publication No. 63-50391 discloses a single crystal pulling apparatus capable of increasing the pulling speed of a single crystal and improving the degree of prevention of dislocation due to contamination of a crystal with impurities. The apparatus has a hollow cylinder which surrounds the grown single crystal, whose inner surface provides a radiated heat reflection prevention surface, and whose surface facing a raw melt provides a radiated heat reflection surface. A cooling device is provided around the cylinder.
This conventional apparatus, however, involves the following problems:
2) When vapors of SiO, SiO.sub.2 and Si from the melt are deposited to and solidified on a heat insulator mounted on the outer surface of the cooled cylinder and fall into the melt, the melt (i.e., raw crystal) is contaminated and the progress of the monocrystallization process or conversion of raw crystal into single crystal is prevented. In addition, if graphite fiber is used for the heat insulator, the graphite fiber exposed to the vapors from the melt is deteriorated and is likely to slip off. As a result, progress of the monocrystallization process is prevented.